1. Field of the Invention
This invention relates to the exterior curtainwall construction. The curtainwall system is supported on spaced apart vertical mullions. The vertical mullions are structurally secured to the building at the edges of floor and roof slabs or spandrel beams. This invention is related to the connection means of the vertical mullions.
2. Description of the Prior Art
A curtainwall system is an exterior wall system installed outboard of the building perimeter frame to provide protections against the exterior weather conditions. The exterior wall system is normally supported on spaced apart vertical mullions. The vertical mullions are structurally connected to the building perimeter frame to provide the following two structural functions. The first structural function is to support the dead weight of the exterior wall system. The second structural function is to resist the inward or outward horizontal reaction forces transmitted from the exterior wall system due to wind loads. The available locations for connecting the mullions to the building are located along the edges of the roof and floor slabs. Aside from the wind load resisting requirement, the functional requirements of the exterior wall system include watertight performance and maintaining a certain degree of air tightness for the consideration of thermal efficiency. To maintain these functional requirements, the relative movements of the exterior wall joints must be minimized to be within the design tolerance. Since the mullions are structurally connected to the building along the edges of the floor and roof slabs, the effect of the relative deflection along the edges of the slabs between floors due to the variable live loads must be considered. Normally the floor slab is designed for a maximum allowable deflection of 1/360 of the span. For a commonly used span of 30 feet (9.14 m), the allowable maximum deflection is one inch (25.4 mm). This amount of deflection will become the relative movement between the floors if one of the floor is fully loaded and the other floor is empty. If this amount of relative movement is transmitted into the exterior wall system, the exterior wall joint can hardly be designed to accomodate the movement while maintaining the functional requirements. Therefore, it is commonly required that the connection means of the vertical mullions must be free from the effect of slab deflection.
There are three known methods for dealing with the floor deflection problem. The first method is to fasten the mullions to an independent horizontal truss spanning between building perimeter columns. Of course, this method is very expensive and thus it is seldomly used. The seocnd method is to hang the mullions on relatively rigid roof spandrel trusses or beams and to provide vertically slidabe connections at the floor levels. However, this method makes the mullion butt joint design difficult and expensive and thus it is seldomly used on buldings of more than two story high. The third method is to vertically support the mullion at the base floor level which is commonly a rigid bearing wall structure and to provide vertically slidable connections at the roof and the other floor levels. The third method is most popular in the curtainwall construction. The vertically slidable connection mean of the prior art design normally uses two spaced apart structural angle clips allowing the mullion to go in between the two confronting legs of the structural angles. The structural angle clips are structurally connected to the edge of the slab or the spandrel beam. The clips and the mullion are fastened together using structural bolts penetrating through the mullion and the protruding legs of the clips where vertically elongated bolt holes are provided in the angle clips. The working principle is explained as follows. When the slab undergoes deflection, the clips which are rigidly connected to the slab will slide downwardly relative to the mullion within the elongated hole without forcing the mullion downwardly. The angle clips are normally installed by welding before the installation of the mullion. The following practical considerations must be given in the execution of the vertically slidable connection design.
(1). In providing the gap between the two clips, the following tolerances must be considered. For the clips, they include the locational tolerance, the out of plumb tolerance among different floor levels, and the rotational tolerance of each individual clip. For the mullion, it is the side bow tolerance. Adding all the above tolerances together, it is normally required to design the gap to be about one inch (25.4 mm) wider than the width of the mullion.
(2). In providing the location of the bolt hole in the clips, the out of plumb tolerance among the slab edges of all floors must be considered. This tolerance normally requires about one inch (25.4 mm) inward and outward adjustability of the mullion location. This inward and outward adjustability can be referred to as the depthwise adjustability. Due to this tolerance requirement, the bolt holes through the mullion must be drilled in the field.
The drawbacks of the prior art slidable connection means are itemized as follows.
(1). The slidability of the elongated bolt hole may be impaired by over tightening of the bolt or rust binding. It is extremely difficult to provide a field guidence as to the proper tightness requirement for the connection bolt.
(2). Before the mullin is bolted in place, the mullion is floating in the air within the oversized gap between the clips. Since there is no inward or outward adjustability once the bolt holes on the mullion have been drilled, the final plumbing of the outer face of the mullion must be done before drilling the bolt holes. To secure the mullion in position for the bolt hole drilling is difficult involving temperary shimming operation. The shimming operation or the vibration during hole drilling may throw the mulliom out of plumb again, therefore, a tedious procedure of checking and rechecking is normally encountered.
(3). To plumb the side face of the mullion, shims on both sides between the mullion and the clips must be used. However, the difference in the thickness of the shim between the two sides affects the horizontal distance between the mullions. Therefore, the side face plumbing and the adjustment of the horizontal distance must be executed concurrently. This requires at least a three-man crew in performing the task.
(4). Theoretically the location of the bolt hole in the mullion at the floor level should be located at the bottom of the elongated hole during erection to allow for free downward movement of slab due to live loads. However, some upward movement is possible due to floor vibration or thermal differential between the erection condition and the service condition, therefore, the bolt hole should be located slightly above the bottom of the elongated hole. Significant upward movements exist at the roof level due to the uplifting wind forces. Therefore, at the roof level, the bolt hole in the mullion should be located in the middle region of the elongated hole on the clips. Field error is very likely due to the different locational requirements.
(5). In a utilized erection where preassembled wall segment on multiple mullions is lifted into place and then connected to the clips. Due to the sealing requirement along the vertical joint between two wall segments, it normally requires a parallel horizontal movement to engage and to form a sealed vertical joint. To accomodate this lateral side engagement motion, it would require a wide gap between the two clips making the design for the connection strength difficult if not impossible due to bending on the bolt. To install the clip on the engaging side after the engagement of of each wall segment is cost prohibitive due to the requirement of performing welding on all floor levels after the engagement of each wall segment. In addition, sparks during welding may damage the finishes of the preassembled wall segment. In the prior art system utilizing unitized erection procedure, much more elaborated connection system including prewelded horizontal seating clip on the building frame and preassembled slidable connection assemblies on the mullions of each wall segment. Upon the engagement of the wall segment, the connection assemblies on the mullions are then bolted to the seating clips on the building frame. The clip design to allow positioning and rotational adjustments is always expensive and difficult to execute in the field.